Vehicle systems and components are tested during the development and manufacturing thereof using various techniques. The systems and components undergo static, quasi-static, and dynamic testing to meet and exceed various durability, lifespan, and performance requirements. The testing techniques are used to simulate a working environment and to assure that the systems and/or components under test can survive and perform as desired within that environment.
In performing such tests various actuators are utilized. The actuators apply force on the systems and or components under test. It is desirable that the applied force be accurately controlled. It is also desirable that the actuators when appropriate be capable of following the motion of a device without imposing uncharacteristical forces or in effect restricting the motion of that device. For example, when testing an airfoil or wing of an aircraft tens to hundreds of actuators may be coupled to both sides of that wing. The actuators must work in unison to flex the structure and cause accelerated fatigue. Some tests require high cycle rates, which can be difficult to follow due to changing dynamics experienced by the actuators. As an example, when the wing is being flexed in an upward direction, the actuators coupled to the topside of that wing ought to follow the motion and velocity of that wing without uncharacteristically restricting its movement while the bottom side actuators are applying force in an upward direction. In other words, the non-acting or force-imposing actuators should not impose forces on the wing that would not normally be experienced in a normal operating environment.
Dynamic load control involves the accurate application of compressive and tensile forces to a moving object. When this load is applied using hydraulic actuators and servo valves with classical control methods, the resulting accuracy is typically unacceptable at maximum rated pressures and flows. There is inaccuracy in the applied force relative to the commanded force signals due to motion of the object. This inaccuracy is affected to such a degree that the motion of the object must be drastically reduced to obtain the desired force control accuracy.
Force control systems can also experience lack in symmetry in system polarity. This is especially true when utilizing single ended actuators, in which the amount of lack in symmetry is significant. The lack of symmetry can be due to differences in tubing connections, servo valves, and other components and devices.
Many force control systems, such as lag-lead type, lead-lag type, and pole-zero placement type systems, tend to increase system loop gain in order to reduce steady state error and thus compensate for the motion of the tested object. However, the increased gain only marginally reduces the error and leads to limit cycling or oscillation and instability in the force control loop. Even with the increased gain, these types of force control systems are not robust when null shifts occur in the equipment. Null shifts cause a proportionate amount of force errors to accumulate. Thus, these systems are partially uncontrolled systems and are only marginally stable and as error increases the systems become unstable and are sensitive to null shifts in the hardware.
Another method used to increase system robustness and reduce steady state error is error integration, which is successful in slower systems that do not have backlash and other non-linearities due to mechanical linkages. Backlash can be introduced from, for example, pin-slop. Although useful for static conditions, error integration can cause instability during transitions in force polarity. The integration process increases the settling time required for the actuator to apply an accurate load. This added time constraint reduces the benefit of using integral compensation on durability tests that have fast changing set point commands.
Yet another method used to reduce steady state error is referred to as damping derived from the force signal. This method tends to be slow, which results in larger errors for longer time intervals. The errors are larger due to the increased time for the damping algorithm to correct for the velocity.
Thus, there exists a need for an improved force control system and technique of applying controlled forces on an object under test without the steady state errors, increased settling time, and other associated disadvantages.